Chapter overview: endocrine coordination
Neural coordination is rapid but short-lived and nerve fibres do not reach every cell. Hormones provide continuous chemical coordination and integration; neural and endocrine systems work together to regulate physiology.
Hormones are non-nutrient chemicals produced in trace amounts that act as intercellular messengers. Endocrine glands lack ducts, so their secretions enter blood directly.
Organised endocrine bodies include hypothalamus, pituitary, pineal, thyroid, parathyroid, thymus, adrenal glands, pancreas and gonads. Heart, kidney, gastrointestinal tract and other tissues also make hormones.
Hypothalamus and pituitary gland
Hypothalamus is the basal part of diencephalon. Its neurosecretory nuclei make releasing hormones that stimulate pituitary secretion and inhibiting hormones that suppress it. Hormones reach anterior pituitary through portal circulation; posterior pituitary is directly controlled neurally.
| Part / hormone | Function |
|---|---|
| Anterior pituitary / pars distalis | Secretes GH, PRL, TSH, ACTH, LH and FSH. |
| Pars intermedia | Secretes MSH; almost merged with pars distalis in humans. |
| Posterior pituitary / pars nervosa | Stores and releases hypothalamus-made oxytocin and vasopressin (ADH). |
| GH | Excess: gigantism; low secretion: pituitary dwarfism; adult excess: acromegaly. |
| PRL | Mammary-gland growth and milk formation. |
| TSH / ACTH | TSH stimulates thyroid hormones; ACTH stimulates adrenal-cortex glucocorticoids. |
| LH / FSH | Gonadotrophins. In males, LH stimulates androgen secretion and FSH with androgens supports spermatogenesis. In females, FSH develops follicles; LH causes ovulation and maintains corpus luteum. |
| MSH | Acts on melanocytes and regulates skin pigmentation. |
| Oxytocin / ADH | Oxytocin causes uterine contraction and milk ejection. ADH promotes distal tubular water/electrolyte resorption; impaired ADH causes diabetes insipidus. |
Pineal, thyroid and parathyroid glands
| Gland / hormone | Key actions and disorders |
|---|---|
| Pineal / melatonin | Maintains 24-hour rhythm including sleep-wake cycle and temperature; influences metabolism, pigmentation, menstrual cycle and defence capacity. |
| Thyroid / T3, T4 | Iodine-dependent. Regulate BMR, RBC formation, carbohydrate/protein/fat metabolism, and water-electrolyte balance. Iodine deficiency causes hypothyroidism and goitre. |
| Hypothyroidism | During pregnancy, may cause cretinism: stunted growth, mental retardation, low IQ, abnormal skin and deaf-mutism. May make menstrual cycle irregular in adult women. |
| Hyperthyroidism | Excess thyroid hormone, often with cancer/nodules. Exophthalmic goitre / Graves’ disease has enlarged thyroid, protruding eyeballs, high BMR and weight loss. |
| Thyroid / thyrocalcitonin (TCT) | Protein hormone regulating blood calcium, chiefly decreasing it. |
| Parathyroid / PTH | Four glands on thyroid’s back. PTH raises blood Ca2+: bone resorption, renal Ca2+ reabsorption and increased intestinal absorption; hypercalcemic hormone. |
Thymus and adrenal glands
| Gland / region | Hormones and functions |
|---|---|
| Thymus / thymosins | Behind sternum between lungs. Promotes T-lymphocyte differentiation (cell-mediated immunity) and antibody production (humoral immunity). Degenerates in old age, weakening immune response. |
| Adrenal medulla / catecholamines | Adrenaline (epinephrine) and noradrenaline: emergency fight-or-flight hormones. Increase alertness, pupil dilation, piloerection, sweating, heartbeat, contraction strength and respiration; stimulate glycogen, lipid and protein breakdown. |
| Adrenal cortex / glucocorticoids | Cortisol stimulates gluconeogenesis, lipolysis and proteolysis; maintains cardiovascular and renal function, increases RBC production, suppresses inflammation and immune response. |
| Adrenal cortex / mineralocorticoids | Aldosterone increases renal Na+ and water reabsorption and K+/phosphate excretion; maintains electrolytes, body-fluid volume, osmotic pressure and BP. |
| Adrenal cortex / androgens | Small amount contributes to axial, pubic and facial hair at puberty. |
| Disorder | Underproduction by adrenal cortex causes Addison’s disease with altered carbohydrate metabolism, acute weakness and fatigue. |
Pancreas: insulin and glucagon
Pancreas is a composite exocrine and endocrine gland. Its 1-2 million islets of Langerhans form only 1-2% of pancreatic tissue.
| Feature | Glucagon | Insulin |
|---|---|---|
| Source | Alpha cells | Beta cells |
| Main target | Hepatocytes | Hepatocytes and adipocytes |
| Action | Glycogenolysis and gluconeogenesis; reduces cellular glucose uptake/utilisation. | Enhances cellular glucose uptake/utilisation and glycogenesis. |
| Blood-glucose effect | Raises glucose: hyperglycemic hormone. | Lowers glucose: hypoglycemic hormone. |
Diabetes mellitus Prolonged hyperglycemia leads to glucose loss in urine and ketone-body formation. Insulin therapy treats diabetic patients.
Gonads: testis and ovary
| Gonad | Hormone source | Important actions |
|---|---|---|
| Testis | Leydig / interstitial cells produce androgens, chiefly testosterone. | Development and function of male accessory organs; facial/axillary hair, muscular growth, low voice, aggressiveness, spermatogenesis, libido, and anabolic effects on protein/carbohydrate metabolism. |
| Ovary: follicles | Growing follicles secrete estrogens. | Female accessory-organ and secondary-sex-character growth, follicle development, mammary development and sexual behaviour. |
| Ovary: corpus luteum | After ovulation, ruptured follicle forms corpus luteum that mainly secretes progesterone. | Supports pregnancy; promotes mammary alveoli formation and milk secretion. |
19.3 Hormones from heart, kidney and GI tract
| Source / hormone | Function |
|---|---|
| Atrial wall / ANF | High blood pressure releases atrial natriuretic factor; it dilates blood vessels and lowers BP. |
| Kidney / erythropoietin | Stimulates erythropoiesis (RBC formation). |
| GI tract / gastrin | Stimulates gastric HCl and pepsinogen secretion. |
| GI tract / secretin | Stimulates exocrine pancreas to release water and bicarbonate ions. |
| GI tract / CCK | Stimulates pancreatic enzymes and bile secretion from gall bladder. |
| GI tract / GIP | Inhibits gastric secretion and motility. |
| Growth factors | From several non-endocrine tissues; essential for normal tissue growth and repair/regeneration. |
19.4 Mechanism of hormone action
Hormones act only on target tissues bearing their specific receptors. Hormone-receptor complex formation triggers biochemical changes that regulate metabolism and physiology.
| Feature | Membrane-bound receptor | Intracellular / nuclear receptor |
|---|---|---|
| Hormone relationship | Hormone normally does not enter target cell. | Hormone enters and binds receptor inside target cell, usually nucleus. |
| Mechanism | Generates second messengers such as cyclic AMP, IP3 and Ca2+. | Hormone-receptor complex regulates gene expression or chromosome function. |
| Examples | Typically peptide, polypeptide and protein hormones. | Steroids and iodothyronines. |
Chemical classes Peptide/protein hormones: insulin, glucagon, pituitary and hypothalamic hormones. Steroids: cortisol, testosterone, estradiol, progesterone. Iodothyronines: thyroid hormones. Amino-acid derivatives: epinephrine.
NCERT revision prompts
- Define endocrine gland and hormone; contrast neural and endocrine coordination.
- Trace hypothalamic regulation of anterior and posterior pituitary.
- List pituitary hormones and relate GH/ADH imbalance to disorders.
- Compare thyroid, parathyroid and calcitonin roles in calcium metabolism.
- Compare adrenal medulla and cortex, including emergency response and corticoid roles.
- Compare insulin and glucagon in glucose homeostasis.
- State androgen, estrogen and progesterone actions.
- Match ANF, erythropoietin and gastrointestinal hormones to their functions.
- Compare membrane and intracellular receptor mechanisms.